1. Field of the Invention
The present invention relates to a light-emitting device and a method of fabricating the same.
2. Description of the Related Art
(First Invention)
A light-emitting device having the light emitting layer portion composed of (AlxGa1−x)yIn1−yP (where, 0≦x≦1, 0≦y≦1; also referred to as AlGaInP, hereinafter) alloy can be realized as a high-luminance device by adopting a double heterostructure in which a thin AlGaInP active layer is sandwiched by an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both of which having a band gap energy larger than that of the active layer. Recent efforts have also succeeded in realizing a blue light-emitting device in which a similar double heterostructure is formed using InxGayAl1−x−yN (where, 0≦x≦1, 0≦y≦1, x+y≦1; also referred to as InGaAlN, hereinafter).
In an exemplary case of an AlGaInP light-emitting device, the double-heterostructured, light-emitting layer portion is formed on an n-type GaAs substrate by hetero-formation, in which an n-type GaAs buffer layer, an n-type AlGaInP cladding layer, an AlGaInP active layer, and a p-type AlGaInP cladding layer are sequentially stacked in this order. Current supply to the light-emitting layer portion is achieved through a metal electrode formed on the surface of the device. The metal electrode herein serves as a light interceptor, so that it is formed so as to cover only a center portion of the main surface of the light-emitting layer portion, so as to extract light from the surrounding non-electrode-forming area.
In this case, it is advantageous to reduce the area of the metal electrode as possible in view of improving the light extraction efficiency, because the are a of the light extraction region formed around the electrode can be enlarged. Although some conventional efforts have been directed to increase the amount of light extraction by effectively spreading the current throughout the device through improving shape of the electrode, increase in the electrode area is inevitable in anyway, and this causes a dilemma that the amount of light extraction is restricted instead by decrease in the narrowing of the light extraction area. What is worse, carrier concentration of dopant in the cladding layer, or conductivity, is suppressed somewhat to a lower level so as to optimize emission recombination of the carriers within the active layer, and this makes the current less likely to be spread in the in-plane direction. This undesirably results in concentration of current density to the electrode-formed portion, and reduction in the substantial amount of light extraction from the light extraction area. One known countermeasure relates to a method of forming a current-spreading layer having a raised carrier concentration and a low resistivity between the cladding layer and the electrode. The current-spreading layer necessarily has a thickness of at least 5 μm to 10 μm to ensure a sufficient current spreading efficiency, and is formed by the metal organic vapor phase epitaxy (MOVPE) process or the liquid phase epitaxy (LPE) process.
On the other hand, a variety of device structures allowing light extraction from both surfaces of the light-emitting layer portion are proposed in view of improving the light extraction efficiency. In the AlGaInP light-emitting device, a GaAs substrate is used as a growth substrate for light-emitting layer portion, where GaAs has a large light absorption in the emission wavelength region of the AlGaInP light-emitting layer portion. Japanese Laid-Open Patent Publication No. 2001-68731 discloses a method in which the GaAs substrate is once separated, and a transparent conductive substrate such as being composed of GaP or the like, which is transparent in the emission wavelength region, is newly bonded so as to make it possible to extract light from both surfaces of the light-emitting layer portion. The publication describes that the transparent conductive substrate is bonded to the light-emitting layer portion while placing a conductive oxide layer such as being composed of ITO (indium tin oxide) in between.
In the light-emitting device disclosed in the above publication, the light-emitting layer portion and the transparent conductive electrodes are directly bonded through the conductive oxide layer in contact with both of them. The conductive oxide layer such as being composed of ITO is, however, shows a large contact resistance with the compound semiconductor composing the light-emitting layer portion or the transparent conductive substrate, and the above-described direct bonding raises a problem in that the device will no more be operable at an appropriate operational voltage due to an excessively large forward series resistance.
Another disadvantage resides in that the conductive oxide layer in the above publication is formed by coating and baking of a colloidal solution containing ITO fine particles, where the conductive oxide layer formed by this method only shows a small bonding strength with respect to the transparent conductive substrate, and likely to cause separation.
(Second Invention)
There are also proposed a variety of device structures in which the light emitted towards the back surface of the light-emitting layer portion is reflected towards the light extraction surface side to thereby raise the light extraction efficiency. The reflective layer available for these structures is typically composed of a distributed Bragg reflector (DBR) in which a number of layers differing in the refractive indices are stacked (see Japanese Laid-Open Patent Publication No. 7-66455, for example). Such DBR can be formed by hetero epitaxial growth of a thin compound semiconductor layer on the substrate prior to formation of the light-emitting layer portion, but the largeness in the number of layers to be grown tends to raise the cost. It is also disadvantageous that the DBR, making use of difference in the refractive indices of the stacked semiconductor layers, can reflect the light incident at only a limited range of angle, so that a large increase in the light extraction efficiency is not expectable in principle. On the other hand, Japanese Laid-Open Patent Publication No. 2001-339100 discloses a device structure in which a metal reflective layer is interposed between the substrate and the light-emitting layer portion. The metal layer is advantageous in having a large reflectivity, and the reflectivity is less dependent on the angle of incidence and wavelength. It is, however, necessary in the method of fabrication disclosed in the publication that the light-emitting layer portion is formed by hetero-epitaxial growth on the light-emitting-layer-growing substrate (also referred to as “growth substrate”), typically composed of GaAs, and that another conductive substrate is bonded on the main surface of the light-emitting layer portion opposite to the growth substrate, while placing the metal layer in between. The light-emitting-layer-growing substrate is separated after the bonding, and the resultant separation surface side is used as the light extraction surface. In order to configure the device so that the current-spreading layer is formed on the separation surface side in this case, it is necessary to preliminarily form a semiconductor layer intended for becoming the current-spreading layer on the light-emitting-layer-growing substrate in advance to the hetero-epitaxial growth of the light-emitting layer portion.
The current-spreading layer in this method, formed in the early stage of the growth on the light-emitting-layer-growing substrate, must however satisfy a strict lattice matching condition as a consequence, and is restricted by various aspects. For instance, for the case where a GaAs substrate is used, AlGaAs is one of few candidates of a compound semiconductor for forming the current spreading layer capable of attaining lattice matching therewith. AlGaAs has, however, only a relatively small band gap energy, and is likely to cause light absorption (in particular at a yellowish green emission wavelength around 560 nm). It is also disadvantageous that Al contained therein is very likely to be oxidized at high temperatures during epitaxial growth of the light-emitting layer formed thereafter. It is also disadvantageous that the epitaxial growth of the AlGaAs layer inevitably increases the number of process steps using expensive MOVPE process. A large disadvantage resides in that a considerably thick current-spreading layer will be necessary to obtain a sufficiently large current spreading effect. Still another disadvantage resides in that the current spreading layer has a considerably large dopant concentration, and this is causative of quality degradation of the light-emitting layer portion epitaxially grown thereon, diffusion of the dopant from the current-spreading layer to the light-emitting layer portion, and auto-doping.
(Third Invention)
As a result of advancement which has been made for a long period on the materials and device structures adoptable to light-emitting devices such as light-emitting diodes or semiconductor lasers, the light-electricity conversion efficiency within the devices has gradually approached the theoretical limits. In order to achieve the devices having a further high luminance, it may therefore be very important to raise the light extraction efficiency. In an exemplary light-emitting device having the light-emitting layer portion thereof composed of an AlGaInP alloy, a high luminance can be realized by adopting a double heterostructure in which a thin AlGaInP (or GaInP) active layer is sandwiched by an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both of which having a band gap energy larger than that of the active layer. Such AlGaInP double heterostructure can be obtained by epitaxially growing the individual layers composed of AlGaInP alloy on a GaAs single crystal substrate by using AlGaInP alloy's lattice matching property with GaAs. As a general practice, the GaAs substrate is often remained intact and used as a device substrate in the light-emitting devices. AlGaInP alloy composing the light-emitting layer portion has, however, a larger band gap energy than GaAs has, and this raises a problem that the emitted light is absorbed by the GaAs substrate so as to make it difficult to attain a sufficient light extraction efficiency. To solve this problem, there is proposed a method in which the a reflective layer composed of a DBR is interposed between the substrate and the light-emitting device (see Japanese Laid-Open Patent Publication No. 7-66455, for example). A large improvement in the light extraction efficiency is, however, not expectable in principle, because this type of reflective layer, making use of difference in the refractive indices of the stacked semiconductor layers, can reflect the light incident at only a limited range of angle.
Various publications including Japanese Laid-Open Patent Publication No. 2001-339100 has proposed a technique in which the GaAs substrate used for the growth is separated, and a conductive substrate for reinforcement is bonded to the separation surface while placing a metal layer such as an Au layer, also available as a reflective layer, in between. The metal layer is advantageous in having a large reflectivity, and only a small dependence on the angle of incidence and wavelength.
When the metal layer is bonded to the light-emitting layer portion, it is necessary to place an alloyed layer for reducing contact resistance in between. The alloyed layer can be formed by forming a metal layer (referred to as contact metal layer, hereinafter) having a composition capable of distinctively exhibiting a contact resistance reducing effect through alloying with the light-emitting layer portion, and then by annealing for alloying. For example, for the case where the metal layer is formed on the n-type cladding layer side of the AlGaInP light-emitting layer portion, the contact metal layer can be composed of AuGe alloy and so forth. A problem may, however, arise in that the area where the alloying proceeded to a certain degree considerably lowers the reflectivity, so that an effect of raising the light extraction efficiency through reflection on the metal layer is not always ensured to a satisfactory degree.
(Fourth Invention)
The method disclosed in Japanese Laid-Open Patent Publication No. 2001-339100 in which the Au layer as the reflective layer is bonded to the light-emitting layer portion tends to cause separation or lowering in the reflectivity during the bonding. In particular for the case where a metallurgical reaction is likely to proceed during annealing for the bonding between the substrate (particularly Si substrate) and the Au layer, this problem becomes more distinctive.
A first subject of the first invention is therefore to provide a light-emitting device which is producible at low costs, has a low series resistance, and shows a sufficient emission efficiency despite it has a thick current-spreading layer, and to provide a method of fabricating such light-emitting device. A second subject is to provide a method of fabricating a light-emitting device, capable of allowing tight bonding of a transparent conductive semiconductor substrate, used in place of the current-spreading layer, to the light-emitting layer portion.
A subject of the second invention is to provide a method of light-emitting device in which the light-emitting device is fabricated by separating the growth substrate for the light-emitting layer from the light-emitting layer portion, which can make it no more necessary to previously form a thick current-spreading layer on the separation surface side which serves as the light extraction surface, and can fabricate the light-emitting device capable of keeping a large current spreading effect, and is to provide a light-emitting device producible by this method.
A subject of the third invention is to provide a method of fabricating a light-emitting device which can attain a desirable light extraction efficiency from the device by making use of a metal layer as a reflective layer, and successfully excludes a fear of decrease in the reflectivity due to alloying between the metal layer and the light-emitting layer portion, and is to provide a light-emitting device producible by this method.
A subject of the fourth invention is to provide a light-emitting device configured so as to bond the light-emitting layer portion and the device substrate while placing the metal layer in between, in which a metallurgical reaction between the device substrate and the metal layer during annealing for the bonding is successfully prevented, and is consequently less causative of failures ascribable to lowering of the bonding strength or reflectivity due to the reaction, an is to provide a method of fabricating such light-emitting device.